1. Field of the Invention
The present invention relates to an antenna, and in particular to an antenna which is capable of operating in multiple frequency bands.
2. Description of the Prior Art
In recent years, most wireless local area networks (WLANs) use for both 802.11a and 802.11b. The 802.11b standard runs in the 2.4 GHz frequency band. The wireless 802.11a standard runs in the 5 GHz spectrum, from 5.15 to 5.825 GHz, comprising 5.15-5.35 GHz, 5.47-5.725 GHz and 5.725-5.825 GHz frequency bands. Thus, antennas, which can operate in both 2.4 GHz and 5 GHz, become increasingly popular.
U.S. Pat. No. 6,166,694, issued to Ying on Dec. 26, 2000, discloses a conventional antenna. The conventional antenna comprises a first spiral arm and a second spiral arm which are carried by a dielectric substrate. The first spiral arm is sized to function as a first planar inverted-F antenna (PIFA) operating in a first frequency band. The second spiral arm is sized to function as a second PIFA operating in a second frequency band. The conventional antenna forms a matching bridge which is positioned between a feeding pin and a grounding post. By adjusted the length of the matching bridge, the matching of the conventional antenna can be changed. Changing the location of the grounding post can adjust the length of the matching bridge. However, because the grounding post is immovable, changing the location of the grounding post is inconvenient. Therefore, the matching of the antenna cannot be conveniently changed.
U.S. Pat. No. 6,297,776 discloses a conventional PIFA. The conventional PIFA comprises a radiator having a free end fold towards a ground portion. The folded free end of the radiator is capacitively coupled to the ground portion, thereby controlling the characteristics of the PIFA. However, the conventional PIFA operates only in a single frequency band, which cannot comply with dual-band or multi-band operating requirement.
Hence, an improved antenna is desired to overcome the above-mentioned shortcomings of existing antennas.